Cell cultures of chick embryo sympathetic neurons and of an NGF-responsive clonal cell line of rat pheochromocytoma cells will be used as model systems for neurochemical studies. The former system is one in which the differentiation of a purified population of neurons can be studied in vitro. The latter system is a newly developed clonal cell line which resembles adrenal chromaffin cells and which in the presence of nerve growth factor (NGF), express neuronal properties such as neurite outgrowth and electrical excitability. We shall use these systems to study (a) the mechanisms of action of NGF in initiating and maintaining neurite outgrowth and regeneration, (b) in a cause and effect relationship, the changes which occur in neuronal protein composition and synthesis during differentiation and (c) the regulation during development of two proteins of known function, acetylcholinesterase and the acetylcholine receptor. Experiments will also be carried out to further characterize the effects of NGF on the differentiation of the pheochromocytoma cell line and to further establish this line as a generally useful model system for neurochemical studies.